Wind turbines have received increased attention as a renewable energy source. Wind turbines use the wind to generate electricity. The wind turns multiple blades connected to a rotor. The spin of the blades caused by the wind spins a shaft of the rotor, which connects to a generator that generates electricity. Certain wind turbines include a doubly fed induction generator (DFIG) to convert wind energy into electrical power suitable for output to an electrical grid. DFIGs are typically connected to a converter that regulates the flow of electrical power between the DFIG and the grid. More particularly, the converter allows the wind turbine to output electrical power at the grid frequency regardless of the rotational speed of the wind turbine blades.
A typical DFIG system includes a wind driven DFIG having a rotor and a stator. The stator of the DFIG is coupled to the electrical grid through a stator bus. A power converter is used to couple the rotor of the DFIG to the electrical grid. The power converter can be a two-stage power converter including both a rotor side converter and a line side converter. The rotor side converter can receive alternating current (AC) power from the rotor via a rotor bus and can convert the AC power to a DC power. The line side converter can then convert the DC power to AC power having a suitable output frequency, such as the grid frequency. The AC power is provided to the electrical grid via a line bus. An auxiliary power feed can be coupled to the line bus to provide power for components used in the wind turbine system, such as fans, pumps, motors, and other components of the wind turbine system.
A typical DFIG system includes a two-winding transformer having a high voltage primary (e.g. greater than 12 KVAC) and a low voltage secondary (e.g. 575 VAC, 690 VAC, etc.) to couple the DFIG system to the electrical grid. The high voltage primary can be coupled to the high voltage electrical grid. The stator bus providing AC power from the stator of the DFIG and the line bus providing AC power from the power converter can be coupled to the low voltage secondary. In this system, the output power of the stator and the output power of the power converter are operated at the same voltage and combined into the single transformer secondary winding at the low voltage.
More recently, DFIG systems have included a three winding transformer to couple the DFIG system to the electrical grid. The three winding transformer can have a high voltage (e.g. greater than 12 KVAC) primary winding coupled to the electrical grid, a medium voltage (e.g. 6 KVAC) secondary winding coupled to the stator bus, and a low voltage (e.g. 575 VAC, 690 VAC, etc.) auxiliary winding coupled to the line bus. The three winding transformer arrangement can be preferred in increased output power systems (e.g. 3 MW systems) as it reduces the current in the stator bus and other components on the stator side of the DFIG.
During operation of wind turbine systems, including DFIG systems, various grid faults can occur, which result in a disconnect between generation of power by the wind turbine and receipt of that power by the grid. This can result in excessive energy in the power converter, which can cause damage to the converter.
Various approaches have been utilized to reduce the risk of overvoltage conditions in power converters. For example, crowbars have been utilized to prevent excess energy from reaching the power converter when grid faults occur. More recently, dynamic brake systems have been utilized. Conventional dynamic brake systems include a resistor in series with a switch, such as an insulated-gate bipolar transistor (IGBT), and absorb excess energy in the converter when gated on during when a grid fault occurs.
However, conventional dynamic brake systems are not without drawbacks. For example, when a dynamic brake is gated on, the line side of the power converter is not protected from excess energy. This excess line side energy can be transmitted through, and damage, the line side of the converter. Further, in some cases during operation of dynamic brakes, a dynamic brake may be gated off due to increased temperature conditions before sufficient energy has been absorbed, thus again risking damage to the power converter.
Accordingly, improved methods for operating wind turbine systems are desired. In particular, improved methods which utilize dynamic brakes and provide reduced risk of power converter damage would be advantageous.